Thunderstorm

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Thunderstorm

A thunderstorm, or an electrical storm, is a strong disturbance in the atmosphere that brings heavy rain, lightning, and thunder to areas from one to hundreds of kilometers across. An average thunderstorm may release as much energy as 10,000,000 kilowatt-hours— a large thunderstorm may release 100 times as much energy.

Thunderstorms are formed when humid air near the surface begins rising and cooling. The rising air forms clouds. Storms develop when the clouds cool enough to bring about the growth of rain droplets or ice crystals. Eventually the growing drops or crystals fall out of the cloud as precipitation. Strong updrafts and downdrafts that are inside a thunderstorm can cause static charges to build up in the cloud. Charges of opposite sign (electrical polarity) accumulate in different parts of the cloud until a spark occurs between them, resulting in the jagged bolts of lightning associated with thunderstorms. Severe thunderstorms may include hail, tornadoes, and damaging straight line winds, making these storms among nature’s most destructive.

Thunderstorm development

Thunderstorms develop in the same process that forms the puffy clouds of summer skies, cumulus clouds. These clouds form when humid air (that is, air with an abundance of water vapor) near the surface is pushed up by being forced over a mountain range, a front, strong solar heating of the surface, or some other means. As the air rises through the atmosphere, it expands and cools. Eventually the rising air cools to the point where its water vapor condenses to form droplets of liquid water. A huge collection of these tiny suspended droplets forms a cloud. At this stage, the rising air is visible as a cumulus cloud, called a convective cloud since it forms by convection (vertical air movement). During fair weather, the convective clouds stop their vertical growth at this point and do not bring rain.

To form a thunderstorm from a convective cloud several conditions are necessary. Most importantly the atmosphere must be unstable. In an unstable atmosphere the air temperature drops rapidly with height, meaning any bubble of air that begins rising and cooling will remain warmer than its surroundings. At every point in its ascent, the rising air acts like a hot air balloon: since it is warmer and less dense than the surrounding air, it continues to rise.

A second requirement for a strong thunderstorm is plenty of humid air. This condition supports the growth of cloud droplets and actually fuels the rising air through latent heat. The water vapor in the air comes from the evaporation of liquid water somewhere—most likely the oceans. To evaporate the water into vapor, energy is required, just as heat must be added to a kettle to make its water boil. This energy carried with the water vapor wherever it goes is latent or hidden heat. If and when the vapor condenses to form liquid water, the latent heat will be released back into the environment. Thus, when the water vapor in rising air condenses to form water droplets, a significant amount of heat is released to the surrounding air. Heating the air makes it less dense and increases the tendency of the air bubble, now a cloud, to rise.

As the air continues to rise and cool, droplets within the cloud begin to grow by coalescence (sticking together). In the clouds of colder climates, droplets may freeze to form ice crystals, which grow as more and more water vapor condenses on them. The droplets or ice crystals, known as precipitation particles, only grow as long as they can be supported by the updrafts. When they grow too large, they begin to fall out of the cloud as drizzle or raindrops. If the updrafts in the cloud are vigorous enough, much larger precipitation will be formed. In a thunderstorm the uplift process is so strong that the cloud grows to the height of the entire lower atmosphere (about 40,000 ft [12 km] above the surface) allowing large raindrops and hailstones to form.

At least two distinct types of thunderstorms can be observed. Over warm humid areas such as the Gulf of Mexico, the air-mass thunderstorm is the most common. These thunderstorms grow from converging cumulus clouds that rise and cool as described above. As the storm matures, rain begins to fall from the upper part of the cloud. The falling precipitation causes downdrafts. This downward moving air eventually overwhelms the rising air. The downdrafts effectively shut off the uplift necessary for the storm to grow, so the storm dissipates as the air sinks and no more rain is formed. These types of thunderstorms are

common over the Florida peninsula, where they bring showers and lightning strikes but rarely any hail or damaging winds, unless frontal action is nearby.

Potentially more severe thunderstorms form in temperate regions such as the central and eastern United States. Called frontal thunderstorms, these storms often form ahead of the advancing edge of a cold air mass (a cold front). In the summer months, the air ahead of the cold front is usually warm humid air that is highly unstable. The denser cold air forces the warmer lighter air ahead of it to rise, forming convective clouds and, eventually, rain. As in an air mass thunderstorm, the falling rain causes downdrafts in the cloud. Unlike the air mass storm, a frontal thunderstorm is arranged so that it is intensified by the downdrafts. The downdrafts become strong gusts of down-flowing air. When they reach the ground, the downdrafts spread out and force more warm humid air to begin rising into the thunderstorm. This condition provides the storm with more latent heat, strengthening the cloud’s updrafts, increasing its wind speeds, and improving the chances of heavy rain and hail. The storm advances into the warm air, vacuuming up humid air, and transforming it into a very organized system of powerful updrafts and down-drafts. After the storm and the front passes, the affected area is often affected by the cold air behind the front where temperatures and humidities are usually much lower.

Hail, lightning, and tornadoes

Strong updrafts in a thunderstorm support the growth of large rain drops and ice crystals. In a severe storm, some of the ice crystals may be dragged down by the downdrafts, then swept up again by updrafts.

Ice particles may be circulated several times through the storm cloud in this manner, picking up water with each cycle. In a process called riming, rainwater freezes onto the ice particles and eventually grows to be large hailstones. Hailstones continue to be recirculated through the cloud until they grow large enough to fall out under their own weight, falling against the strong updrafts. If located in the appropriate part of the storm, hailstones can grow to impressive sizes. Hail as large as 5.5 in (14 cm) in diameter has been recorded.

Another product of the vigorous up and down drafts in the storm cloud is lightning. Lightning is a giant spark caused by a buildup of static electrical charges, a larger version of the spark one gets by touching a metal doorknob after walking across a cloth carpet. By processes that still are not understood fully, thunderstorm clouds build up a large separation of electric charge with positive charges located near the top of the cloud and negative charges concentrated

KEY TERMS

Air-mass thunderstorm —A thunderstorm typical of tropical areas that may produce heavy rain but rarely any hail or tornadoes.

Convective cloud —A cloud formed from the vertical uplift (convection) of surface air.

Frontal thunderstorm —Thunderstorms associated with cold fronts moving through warm humid air.

Latent heat —The heat given off when water vapor condenses to form liquid water.

Precipitation particles —Rain drops or ice crystals that have grown heavy enough to fall out, or precipitate, out of a storm cloud.

Riming —The freezing on contact of raindrops as they are collected by an ice pellet growing to a hailstone.

Unstable atmosphere —The condition of the atmosphere when air temperature drops rapidly with height. Such conditions support rising air and contribute to strong thunderstorms.

near the middle. Usually the cloud base has a smaller pocket of positive charge. Because the charges are separated, huge voltage differences within the cloud and between the cloud base and the ground often result. The voltage difference is equalized suddenly by a bolt of lightning between these areas. The spark heats the air in the lightning channel to over 54,000°F (30,000°C) causing a rapid expansion. The resulting sound is heard as thunder.

Severe thunderstorms also may form tornadoes, columns of air spinning at extremely high wind speeds. Tornadoes pack wind speeds of 220 mph (over 100 m/second) in a small area, making them capable of great destruction.